1. Field of the Invention
The present invention relates to a developing device and toner for use in an electrophotographic apparatus for printing images in accordance with an electrophotographic process.
2. Description of the Related Art
In an electrophotographic apparatus, a photosensitive surface of a photosensitive drum is charged by a charging device at a predetermined potential (e.g. -600 v), and then the photosensitive surface of the photosensitive drum is exposed by an exposure unit in accordance with an image to be printed, thus forming an electrostatic latent image is formed on the photosensitive surface of the photosensitive drum. The electrostatic latent image is developed by a developing device in such a manner that toner is attached to the photosensitive surface of the photosensitive drum in accordance with the electrostatic latent image.
In the developing device, toner is carried and conveyed on a developing roller and put in contact with the surface of the photosensitive drum. When the toner on the developing roller is conveyed, it is formed into a film layer by a developing blade which is made of metal and situated in contact with the developing roller. At the same time, the toner is charged by friction to have the same polarity as that of the potential charged on the photosensitive drum (i.e., negative polarity). A developing bias of a low voltage (e.g. -200 v) and of the same polarity as that of the potential of the photosensitive drum is applied to the developing roller. Toner is selectively attached to the photosensitive drum by the function of the electric field produced in accordance with the electrostatic latent image, the developing bias, and the charge of toner. Specifically, toner is not attached to a non-exposed portion on the photosensitive drum, since the potential of the non-exposed portion is higher than that of the toner. On the other hand, toner is attached to an exposed, discharged portion on the photosensitive drum, since the potential of the exposed portion is lower than that of the toner. Thus, a toner image corresponding to the electrostatic latent image is formed on the surface of the photosensitive drum. The toner image is transferred on a printing paper sheet by a transfer device.
Since image development is performed by the function of the electric field, the development process is not normally carried out if the amount of charges of toner does not reach a predetermined level. A so-called "fogging" phenomenon occurs and toner adheres to the non-exposed region. The quality of printed images is degraded by a fogging. Further, toner is wasted by a fogging.
A rubber roller is employed as the developing roller. In general, as the hardness of the rubber roller increases, the manufacturing precision increases. Therefore, a hardness of a conventional developing roller is high.
However, if the hardness of the developing roller is high, the contact between the developing roller and the metal developing blade tends to unstable. Consequently, some toner particles may pass between the developing roller and developing blade so that they receive little friction and have a small amount of charges may present. A fogging occurs due to these toner particles.